1. Field of the Invention
The present invention relates to exposure control devices for use in photographic apparatus, such as still cameras and the like, and more particularly to shutter control devices for automatically establishing exposure time in accordance with the level of scene illumination.
2. Description of the Prior Art
In many prior art shutter control devices which include timing circuits for establishing shutter speed, the circuit is energized by a battery contained in the camera. Such mechanisms usually include an electromagnetic transducer adapted to close the shutter after a period of time determined by the timing circuit. Although such arrangements are generally satisfactory, they rely on the availability of sufficient power from the battery. When the battery power is inadequate for operation of such systems, either the shutter operates at a fixed, predetermined speed or it remains open indefinitely, depending on the type of shutter control.
In commonly assigned, co-pending U.S. patent application Ser. No. 816,776, entitled PIEZOELECTRIC CAMERA SHUTTER, filed in the names of L. F. Frank and J. K. Lee on July 18, 1977, a shutter control device was disclosed which employed a piezoelectric bimorph for providing electrical energy for powering an electronic exposure control circuit. The shutter control device disclosed in that application represented a significant improvement over previously known mechanisms because piezoelectric devices are more reliable than battery sources and more efficient than conventional, non-battery powered sources known in the prior art for powering camera exposure control systems. In fact, piezoelectric devices have very little resistive loss, and may operate at electro-mechanical conversion efficiencies as high as about 50%.
However, although piezoelectric bimorph devices are efficient generators, the amount of charge which can be generated with each flexure is predetermined for each device, and there are situations wherein the amount of charge would be insufficient to power a camera's automatic exposure control device. By the present invention, a piezoelectric generator is adapted to supply a greater amount of charge by being repeatedly stressed than would be possible by a single stressing of the generator.